Seldom in my years of teaching has an investment of $1.50 returned so much student interest! Shopping at a local nursery, I found small plants of Mimosa pudica, sensitive plant. The price was right, so I bought one, transplanted it to a larger pot, and brought it to school. By the end of the week, other teachers were stopping by my high school classroom, asking to see “The Plant” they had heard about from the students.

This activity, suitable for home or classroom, is a teacher demonstration. Have the students gather around a table in a brightly lit room so that everyone can easily see the plant respond to treatments.

Background

When a predator approaches or the immediate environment becomes hostile, animals can run away or move to a more suitable location. Not so with plants. They are rooted in place and thus have evolved intriguing methods for dealing with such stresses.

Plants do move. Sunflowers follow the sun across the sky, turning their heads to maximize exposure to the light. The stems of plants that have been displaced from the vertical by wind or ice bend upward and resume vertical growth. These are examples of specific movements, or tropisms, called phototropism and gravitropism, respectively.

Some plants display thigmotropism, or movement of plant parts in response to touch. The sensitive plant takes this to the extreme. Touching or shaking the plant, slight warming of the leaves, or subjecting the plant to a lack of water, will all cause the leaflets to fold together and the whole leaf to droop downward temporarily. Even the onset of darkness will cause this response, the leaves remaining folded until dawn.

We can only guess why a plant would invest energy in thigmotropism. Folding and drooping of the leaves may make the plant less noticeable to potential herbivores. And such movement may help the plant conserve water during periods of heat and drought.

Preparation

If you cannot find a sensitive plant at your local garden centers, you can order either seeds or plants from Internet suppliers. (E-mail me, rmanley@ptc-me.net, and I will tell you where I found mine.) Mimosa pudica is a tropical plant and must be grown at temperatures no lower than 62 degrees F.

Place the plant in a bright window for an hour or two before starting the activity. If you must move the plant to another location for the demonstration, do so with minimal disturbance and maintain as much light in the demonstration area as possible. Caution the students not to shake the table or touch the plant!

The demonstration

Begin by gently stroking the mid-vein of a single leaf with your finger or the eraser end of a pencil. The individual leaflets on either side of the mid-vein will fold together. The quickness of the response will depend on the force of the stimulus and strong stroking will cause the stimulus to be transmitted to nearby leaves. Be gentle – you need open leaves for the next step.

Next, light a match and hold it under an open leaf, keeping it as close to the leaf as possible without actually burning the leaf. By the time the match burns too close to your fingers to hold, the plant should be responding, first by folding the leaflets on the warmed leaf together. The entire leaf will then droop downward, followed by folding and drooping of the rest of the leaves.

Explain to your students that these movements are the result of a rapid loss of pressure in specialized cells located at the base of leaflets and entire leaves. Botanists believe that this loss of pressure is due to rapid loss of water from these cells. When the stimulus is removed, the water is slowly reabsorbed and the leaves return to their normal position.